Problem: The scale on a map is 7cm : 5km. If two cities are 28cm apart on the map, what is the actual distance in km between the two cities?
Explanation: The scale means that every 7cm on the map corresponds to an actual distance of 5km. A distance of 28cm on the map is the same as 4 $\cdot$ 7cm. The actual distance between the two cities is 4 $\cdot$ 5km, or 20km.